[PATCH 2/8] block: add BlockParentClass class

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Add a class that will unify block parents for blockdev-replace
functionality we are going to add.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/block-parent.h | 32 +++++++++++++++++
 block/block-parent.c         | 66 ++++++++++++++++++++++++++++++++++++
 block/meson.build            |  1 +
 3 files changed, 99 insertions(+)
 create mode 100644 include/block/block-parent.h
 create mode 100644 block/block-parent.c

diff --git a/include/block/block-parent.h b/include/block/block-parent.h
new file mode 100644
index 0000000000..bd9c9d91e6
--- /dev/null
+++ b/include/block/block-parent.h
@@ -0,0 +1,32 @@
+/*
+ * Block Parent class
+ *
+ * Copyright (c) 2021 Virtuozzo International GmbH.
+ *
+ * Authors:
+ *  Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#ifndef BLOCK_PARENT_H
+#define BLOCK_PARENT_H
+
+#include "block/block.h"
+
+typedef struct BlockParentClass {
+    const char *name;
+
+    int (*find_child)(const char *parent_id, const char *child_name,
+                      BlockDriverState *child_bs, BdrvChild **child,
+                      Error **errp);
+    QTAILQ_ENTRY(BlockParentClass) next;
+} BlockParentClass;
+
+void block_parent_class_register(BlockParentClass *cls);
+
+BdrvChild *block_find_child(const char *parent_id, const char *child_name,
+                            BlockDriverState *child_bs, Error **errp);
+
+#endif /* BLOCK_PARENT_H */
diff --git a/block/block-parent.c b/block/block-parent.c
new file mode 100644
index 0000000000..73b6026c42
--- /dev/null
+++ b/block/block-parent.c
@@ -0,0 +1,66 @@
+/*
+ * Block Parent class
+ *
+ * Copyright (c) 2021 Virtuozzo International GmbH.
+ *
+ * Authors:
+ *  Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+#include "block/block-parent.h"
+#include "qapi/error.h"
+
+static QTAILQ_HEAD(, BlockParentClass) block_parent_classes =
+    QTAILQ_HEAD_INITIALIZER(block_parent_classes);
+
+void block_parent_class_register(BlockParentClass *cls)
+{
+    QTAILQ_INSERT_HEAD(&block_parent_classes, cls, next);
+}
+
+BdrvChild *block_find_child(const char *parent_id, const char *child_name,
+                            BlockDriverState *child_bs, Error **errp)
+{
+    BdrvChild *found_child = NULL;
+    BlockParentClass *found_cls = NULL, *cls;
+
+    QTAILQ_FOREACH(cls, &block_parent_classes, next) {
+        int ret;
+        BdrvChild *c;
+
+        /*
+         * Note that .find_child must fail if parent is found but doesn't have
+         * corresponding child.
+         */
+        ret = cls->find_child(parent_id, child_name, child_bs, &c, errp);
+        if (ret < 0) {
+            return NULL;
+        }
+        if (ret == 0) {
+            continue;
+        }
+
+        if (!found_child) {
+            found_cls = cls;
+            found_child = c;
+            continue;
+        }
+
+        error_setg(errp, "{%s, %s} parent-child pair is ambiguous: it match "
+                   "both %s and %s", parent_id, child_name, found_cls->name,
+                   cls->name);
+        return NULL;
+    }
+
+    if (!found_child) {
+        error_setg(errp, "{%s, %s} parent-child pair not found", parent_id,
+                   child_name);
+        return NULL;
+    }
+
+    return found_child;
+}
diff --git a/block/meson.build b/block/meson.build
index 0450914c7a..5200e0ffce 100644
--- a/block/meson.build
+++ b/block/meson.build
@@ -10,6 +10,7 @@ block_ss.add(files(
   'blkverify.c',
   'block-backend.c',
   'block-copy.c',
+  'block-parent.c',
   'commit.c',
   'copy-on-read.c',
   'preallocate.c',
-- 
2.29.2